Cymbal - Music discovery powered by friends
Spotify - Map shows when two people play same song at same time
humit - A social networking app for music sharing and discovery.
Last.fm - The world's largest online music service. Listen online, find out more about your favourite artists, and get music recommendations, only at Last.fm
Better Notes - Simple notes app that ties notes together with #hashtags
SoundCloud - Enjoy music & follow favourite artists